1. Field of the Invention
Embodiments of the invention generally relate to a pedestal assembly for supporting a substrate in a semiconductor processing chamber.
2. Description of the Related Art
Many semiconductor processes are typically performed in a vacuum environment. For example, physical vapor deposition (PVD) is generally performed in a sealed chamber having a pedestal for supporting the substrate disposed therein. The pedestal typically includes a ceramic support that has electrodes disposed therein to electrostatically hold the substrate against the ceramic support. A target generally comprised of a material to be deposited on the substrate is supported above the substrate, typically fastened to a top of the chamber. A plasma is formed from a gas such as argon that is supplied between the substrate and the target. The target is biased causing ions within the plasma to be accelerated toward the target. The ions impacting the target cause material to become dislodged from the target. The dislodged target material is attracted towards the substrate and deposits a film of material thereon.
Temperature control of the substrate during deposition is critical for good deposition performance. Generally, there are two areas of concern relating to temperature control of the substrate. The first concern is heat transfer between the substrate and the surface of the ceramic support, and the second is the thermal regulation of the ceramic support from within the pedestal. Generally, a backside gas, such as argon or helium, is used as a heat transfer medium between the substrate and the ceramic support.
The second concern is the thermal regulation of the ceramic support. Thermal regulation of the ceramic support from within the pedestal is generally provided by a metallic cooling plate located within the pedestal. In order to maximize the heat transfer between the cooling plate and the ceramic chuck, the mechanical contact area therebetween is maximized to limit the air gaps or voids therebetween. Promoting conductive heat transfer through materials having the solid to solid contact encourages higher heat transfer rates. Generally, thermal conduction through solid materials occurs at a higher rate in contrast to thermal transfer through air gaps or voids, including gaps induced by surface irregularities (flatness, roughness, etc.) in the mating surfaces.
A number of methods have been employed to maximize the solid to solid contact between the ceramic support and the cooling plate. Mechanically attaching the cooling plate to the ceramic support has not been found satisfactory due to the difference in the thermal expansion between the ceramic support and the cooling plate. For example, in applications such as copper PVD, the pedestal is exposed to a temperature range from about xe2x88x9240 to about 200 degrees Celsius. Solders, conductive adhesives and brazing cannot accommodate the difference in thermal expansion between the cooling plate and the ceramic support through such a wide temperature range. Moreover, when the mechanical attachment (i.e., braze, etc.) fails, the cooling plate typically becomes disengaged from the ceramic support, thereby severing the solid to solid conductive path.
Another method of enhancing the thermal conductivity between the cooling plate and the ceramic support is to provide a metallic foil such as aluminum therebetween. However, the foils generally do not lay flat against the cooling plate and the ceramic support surfaces. Moreover, gaps or voids are typically formed as the foil folds upon itself as the foil is compressed between the cooling plate and the ceramic support. The gaps decrease the rate of conductivity across the foil by reducing the solid to solid contact area across the width of the foil. Additionally, the concentrated thermal flux through the portions of the foil having solid to solid contact may be choked if the number of gaps are large, thus leading to a net decrease in the rate of heat transfer over systems not having a foil.
Yet another method of enhancing the thermal conductivity between the cooling plate and the ceramic support is to provide a thermally conductive paste or grease therebetween. However, the rate of conductivity across the grease is typically proportional to the loading between the ceramic support and the cooling plate. In order to provide good thermal conductivity, the load upon the ceramic support is typically high and disadvantageously stresses the ceramic support, thereby making the support susceptible to damage. Moreover, conductive greases are generally not vacuum compatible and are typically limited to applications where temperatures do not exceed about 300 degrees Celsius.
Therefore, there is a need for a pedestal having improved heat transfer characteristics.
A pedestal assembly for supporting a substrate in a semiconductor process chamber is provided. In one embodiment, the pedestal assembly generally includes a ceramic substrate support, a metallic housing and a cooling plate. The ceramic body is coupled to the housing and is adapted to support the substrate on a first surface. The cooling plate is disposed against at least a portion of a second surface of the ceramic substrate support. A conformal graphite interstitial layer is disposed between the cooling plate and the second surface of the ceramic substrate support. The conformal graphite layer provides enhanced thermal conductivity between the cooling plate and the ceramic substrate support over a thermal operating range of the pedestal assembly.
In another embodiment, a pedestal assembly for supporting a substrate in a semiconductor process chamber generally includes a ceramic substrate support, a metallic housing, a cover and a cooling plate. The housing and cover isolate an internal volume from a vacuum environment of the process chamber. The cooling plate is disposed in the internal volume against the cover. The ceramic substrate support is removably coupled to the cover and is adapted to support the substrate on a first surface. A conformal graphite interstitial layer is disposed between the ceramic substrate support and the cover in the vacuum environment of the chamber. Optionally, a second conformal graphite interstitial layer disposed in the internal volume between the cooling plate and the cover.